An abrupt light variation generated during the transmission degrades the reception sensitivity and causes a communication error. To address this, it is desirable to identify a location where the light variation is generated and to cope with the problem. For example, the use of an optical time domain reflectometer (OTDR) enables measurement of a loss generation location of an optical fiber. However, the OTDR is unable to identify a location of an instantaneous variation or a light variation without a loss.
Meanwhile, there is a technique of: inputting light from one terminal of an optical fiber; turning the light back at the other terminal of the optical fiber; monitoring a state of the output light at the one terminal side; and thereby identifying a light variation location from a time difference a light variation generated in the outward path and a light variation generated in the return path (for example, see Japanese Laid-open Patent Publication Nos. 08-136607 and 10-148654).
However, the measurement of a light variation location is impossible unless the light variation generated in the optical fiber of the outward path and the light variation generated in the optical fiber of the return path may be time-resolved. When the light variation generated in the outward path and the light variation generated in the return path are overlapped with each other, the measurement of the light variation locations is failed due to a difficulty in resolving the light variations in the outward path and the return path.